Mice apparently react to visual signs of discomfort in other mice, says a researcher who presented his findings this week at the Society for Neuroscience meeting in Washington.

By Josh Fischman

Washington

If there is human empathy, and no one really doubts that, there should be animal precursors. Charles Darwin predicted this in 1872, in his book The Expression of the Emotions in Man and Animals, but few scientists have pursued the idea. Now a pain researcher in Montreal has stumbled across evidence that mice can feel for other mice—or at least act as if they do.

It was an accidental observation, said Jeffrey S. Mogil, a professor of psychology at McGill University, who described his work this week at the Society for Neuroscience meeting here. Mr. Mogil, whose laboratory studies the genetics of pain responses, had a database of about 8,000 mice who took a "tail withdrawal" test: tails were dipped in mildly heated water, and the mice pulled them out. "We noticed that the second, third, and fourth mice in a cage pulled their tails out much faster than the first mouse," Mr. Mogil said. "So we started to wonder if these animals were somehow communicating with one another."

He wanted to test whether mice would react to another mouse's discomfort. So he tried a new experiment, injecting a mouse's belly with a highly watered-down solution of acetic acid. It caused a stomach ache for a few minutes, and the mouse writhed in a characteristic fashion. "PETA doesn't like it when we call this the 'writhing test,' but we do, and it's both minimally painful and very reliable," Mr. Mogil said.

He measured the movements of one injected mouse against the movements of two mice that were placed together. Two mice moved more, and even more so when they were from the same cage and had previous contact. "It started to look like we were seeing some kind of 'emotional contagion' of pain in mice," Mr. Mogil says. The tests were done on groups of 25 to 30 mice, repeated several times, and the reactions happened more than could be accounted for by chance.

A third test, in which a mouse with discomfort in one body part was placed near another mouse with discomfort in another body part, also seemed to heighten the sensation as compared with how one of those mice acted when alone.

"This message, 'I'm in pain,' seems to be transmitted visually," Mr. Mogil said. He tried blocking sound perception, or smell, and still the mice reacted to one another. It was only when he put an opaque barrier between two mice that the message "I'm in pain" appeared to be shut out. (In earlier work, the McGill group and colleagues at the University of British Columbia identified specific "pain faces" that mice make when suffering; one goal of the research was to develop a way to identify pain in laboratory rodents to ensure that they do not suffer unnecessarily.)

Since he knows how the mice are behaving but not really what is going on inside their heads, Mr. Mogil did not want to go so far as to call these reactions "empathy," instead preferring the term "prosocial behavior." The most prosocial mice in the experiments were females who had been cage mates: They would actually approach the mouse that showed discomfort, and that approach seemed to calm the uncomfortable mouse and reduce the pain-related behavior.

Understanding the connections between familiarity and that prosocial behavior, especially if it holds the roots of empathy, could yield insights into why people help individuals who are hurt in certain situations and ignore them in other circumstances, other scientists have contended. Mr. Mogil thinks it stands to reason that such a complex emotion has simpler animal beginnings.

He also thinks another emotion has prevented many scientists from exploring the issue until recently: fear.

"You go down this road and you run the risk of being tarred with the brush of anthropomorphism," he said. People accuse scientists who take that route of attributing human emotions to animals, and it harms their reputations. The experiments he did are not technically sophisticated, Mr. Mogil added, and could have been done any time during the past half-century, but researchers have shied away.

Noting that most of his own lab is devoted to genetics—"that's what pays the bills"—he said he is willing to devote a little portion of it to exploring a sideline that could illuminate his major focus: where pain comes from.